1. Field of the Invention
This invention generally relates to an electromagnetic coil and the manufacturing apparatus for the same, and more particularly to an electromagnetic coil preferably applied, for example, to an ignition coil for an internal combustion engine or to a compact transformer, and the manufacturing apparatus for such an electromagnetic coil.
2. Related Art
Conventionally, to improve the withstanding voltage and efficiency, a so-called oblique lap winding method shown in FIG. 11 has been preferably used for winding electromagnetic coils applied to ignition coils of internal combustion engines or to compact transformers. "Oblique lap winding", generally designated so in this specification, is one of a plurality of winding methods for winding an electromagnetic coil. As shown in FIG. 11, a wire rod 702 constituting the electromagnetic coil is wound around a cylindrical body of a bobbin 701. More specifically, wire rod 702 is wound and accumulated obliquely at a predetermined gradient angle .theta.0 with respect to the outer cylindrical surface of bobbin 701.
However, when an electromagnetic coil 700 is fabricated by the above-described oblique lap winding method, there is a possibility for wire rod 702 to have a diameter not larger than 0.1 mm, such that the winding collapse may occur when wire rod 702 is wound around bobbin 701. Such a winding collapse tends to occur when a winding pitch P0 of wire rod 702 is set smaller than two times the diameter of wire rod 702, because wire rod 702, when wound on an already wound wire rod 702, possibly pulls away this already wound wire rod 702 from its regular winding position. According to FIG. 11, a reversing-side wire rod 702b is accumulated on an advancing-side wire rod 702a. More specifically, when reversing-side wire rod 702b is wound around bobbin 701, a force acting in the radially inward direction of bobbin 701 forces the reversing-side wire rod 702b to dislocate the already wound advancing-side wire rod 702a in the axial direction of bobbin 701. Hence, the advancing-side wire rod 702a causes undesirable excursion from the predetermined winding position, resulting in the winding collapse.
If such a winding collapse occurs when the wire rod is wound around the bobbin, there will be a possibility that the wire rod dislocated from its regular winding position may approach a wire rod located at a higher-potential winding position. In such a case, corona discharge or electric breakdown may be induced.
To prevent this kind of winding collapse, there are proposed various winding methods for electric winding components as disclosed, for example, in Unexamined Japanese Patent Application No. HEI 2-106910, published in 1990, or in Unexamined Japanese Patent Application No. HEI 2-156513, published in 1990. According to these conventional winding methods, the gradient angle .theta.0 of the wire rod shown in FIG. 11 is, for example, set to an angle of 45.degree. or less, and a winding pitch P0 is set smaller than two times the outer diameter of the wire rod, thereby preventing the winding collapse previously described.
The smaller the gradient angle .theta.0 of wire rod 702 wound around bobbin 701 shown in FIG. 11, the larger the winding number of wire rod 702 per single slant surface. An electric potential becomes large between two neighboring wire rods 702 of two adjacent slant surfaces. This means that the withstanding voltage of wire rod 702 may not be assured or maintained. Hence, it is generally necessary to increase the gradient angle .theta.0 of wire rod 702.
However, according to the winding methods of electric winding components disclosed in the Unexamined Japanese Patent Application No. HEI 2-106910 and the Unexamined Japanese Patent Application No. HEI 2-156513, it was not possible for the wire rod having an outer diameter not larger than 0.1 mm to prevent the above-described winding collapse unless the gradient angle .theta.0 shown in FIG. 11 is set to a small angle.
Furthermore, according to the ignition coil disclosed in Unexamined Japanese Patent Application No. 60-107813, published in 1985, there is proposed a winding method of winding a wire rod by pressing the wire rod from radial directions by a pair of guides made of felt. However, even if this winding method is used, the winding collapse will be caused when the gradient angle .theta.0 shown in FIG. 11 is set to a large angle.
Accordingly, the winding methods for electric winding components disclosed in the Unexamined Japanese Patent Application No. HEI 2-106910 and the Unexamined Japanese Patent Application No. HEI 2-156513 and the ignition coil disclosed in the Unexamined Japanese Patent Application No. 60-107813 have a problem that a sufficient withstand voltage cannot be maintained when the gradient angle .theta.0 is set to a large angle for the wire rod having an outer diameter not larger than 0.1 mm.
Furthermore, when the winding nozzle feeds the wire rod wound around the bobbin, a distance between the winding nozzle and the winding position of the wire rod on the bobbin is believed to be another factor causing winding collapse when the wire rod is wound around the bobbin. As shown in FIG. 11, the distance between winding nozzle 703 and the winding position of the wire rod 702 becomes a minimum distance L01 at the position where wire rod 702 transfers from the layer of reversing-side wire rod 702b to the layer of advancing-side wire rod 702a, and becomes a maximum distance L02 at the position where wire rod 702 transfers from the layer of advancing-side wire rod 702a to the layer of reversing-side wire rod 702b. Therefore, the distance to winding nozzle 703 is small when the winding position of wire rod 702 is located at a radially outside position of bobbin 701. On the other hand, the distance to winding nozzle 703 is large when the winding position of wire rod 702 is located at a radially inside position of bobbin 701. The swingable width of wire rod 702 extracted from winding nozzle 703 varies in proportion to this distance. Accordingly, the swingable width of wire rod 702 is increased with increasing distance between winding nozzle 703 and the winding position of wire rod 702. That is, the swingable width of wire rod 702 increases as the winding position of wire rod 702 approaches toward the outer cylindrical wall of bobbin 701. In other words, the alignment of wire rod 702, when wound around the bobbin 701, tends to be deteriorated in the vicinity of the outer cylindrical wall of bobbin 701. Accordingly, there is a tendency that the winding collapse is possibly induced as wire rod 702 approaches the outer cylindrical wall of bobbin 701.